Auroras are among the most spectacular phenomena in the night sky, thrilling travelers and photographers with wispy displays of green light. But these auroras are not the only thing lighting up the night. Scientists have determined that streaks of purple captured by amateur photographers are not auroras at all, but are rather an entirely new type of celestial phenomenon.
The streaks of strange light have come to be known as STEVEs. The name was originally a reference to the 2006 animated comedy Over The Hedge, but the backronym now stands for Strong Thermal Emission Velocity Enhancement. STEVEs have been known to amateur photographers for decades, but scientists only recently "stumbled upon" them, according to a new paper in Geophysical Research Letters.
A STEVE is an "extremely narrow ribbon of vibrant purple and white hues," according to the paper. What makes this phenomenon different from the aurora borealis is how it's formed. While auroras form when charged particles collide with the upper atmosphere, scientists have yet to figure out exactly what causes a STEVE to occur.
There are additional types of light in the sky as well, referred to as "dayglow" and "nightglow," which refers to lightwaves produced through chemical reactions caused by solar radiation. STEVE isn't explained by that process either.
Scientists are rather puzzled by STEVE. Looking at data from NOAA’s Polar Orbiting Environmental Satellite 17 (POES-17), they think the phenomenon is likely related to a process in the ionosphere, a region of the atmosphere dominated by ions and free electrons. But what is actually occuring is still a mystery.
“Our main conclusion is that STEVE is not an aurora,” says Bea Gallardo-Lacourt, a space physicist at the University of Calgary in Canada and lead author of the paper, in a press statement. “So right now, we know very little about it. And that’s the cool thing, because this has been known by photographers for decades. But for the scientists, it’s completely unknown.”
POES-17 orbits the Earth a little over 14 times a day. By sheer luck it happened to pass directly over a STEVE. With equipment that can measure particle changes within the ionosphere, it could easily detect an aurora or similar activity. But it detected no charged particles raining down from space, as would happen in an aurora. So STEVE must be something else entirely.
“This is really interesting because we haven’t figured it out and when you get a new problem, it’s always exciting,” says Joe Borovsky, a space physicist at the Space Science Institute in Los Alamos, New Mexico. “It’s like you think you know everything and it turns out you don’t.”
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